Dear Lifehacker, I need a laptop, but I don't want to drop a bunch of cash on one. I've seen some lightly used ones on Craigslist that look good, and a few friends have offered to sell me their last-year's models now that they've upgraded. Is there anything wrong with buying last generation's model, slightly used? Anything I should worry about?

Signed, Cautious Buyer

Dear Cautious Buyer, You don't want to get screwed buying a used system, but deciding whether you should is a lot like deciding on a new computer: you have to think about what you're going to use it for, what else you'll need to buy to make it work for you, and how long you want to keep it. In this case, you also have to think about how those things stack up against the cost of a new model. Let's tackle each concern, one at a time.

What Do You Need The Computer For?

Before you buy any computer—new or used—you should ask yourself what you're planning to do with it. Since most used computers are a generation or two behind the most recent models, you have to take special care to make sure you don't need the system for cutting edge work. Here are some criteria to keep in mind:

Portability: Do you travel often? Do you expect to take your work with you? Size and weight are a concern, then: make sure the system you're buying is small enough you can comfortably transport it, but be aware that with portability often (but not always) comes a tradeoff in features and horsepower. Photo by Bruno Cordioli.

Specs: Think about the applications you plan to run. Will you be doing any high-end gaming? Video or audio encoding? Anything else that's CPU or memory-intensive? How much storage will you need? Make sure to consider the specs of the system against the apps you're going to use and ask yourself: Can this computer handle your needs? (Often the answer is "Yes.") If not, is the money you'll save on a used machine worth slower apps, or sticking with earlier versions without new features? (more on this later)

Power: If you're buying a laptop, especially a used one, look up how much a replacement battery will cost, or a spare AC adapter. Depending on how the system was used before it went up for sale, that battery may or may not hold a decent charge, and may be through its recharge cycles and ready to be recycled. This is especially important if you plan to travel, or work in multiple locations. If you're buying a desktop, it's probably not an issue.

All in all, sit down and think about the way you plan to use the system. if your day-to-day computing involves tons of webapps, web services, and few desktop applications, a lightweight system may work for you. If you really need desktop horsepower for gaming, design, and video editing, you want to make sure that used system is up to the challenge.

Make Sure You're Not Better Off Buying New (Or Refurbished!)

The friendly smile of a friend looking to offload a system may be enticing, and you may think you're getting a great deal by taking that laptop off of their hands, but make sure you do your homework before you hand over your cash. Before you say yes, consider these things:

Included Software and Peripherals: Remember, an OEM will include an operating system and usually at least a 1 year warranty. If you're buying refurb or from an authorized reseller, they'll probably include the OEM license for the OS, and offer their own service plan, or toss in a warranty with a service like SquareTrade. See what comes with yours—or how much it would cost to get what you need. The same applies to peripherals like keyboards and mice.

Competitive Pricing: Aside from raw specs and stats, look at the price of the used model, and compare it to the price of a new, comparable system. Remember, used doesn't always equal bargain, even if your Craigslist contact is tossing in some pre-installed software or an external hard drive. Would you spend the same buying new or refurb, perhaps with that previously mentioned warranty?

Price of Upgrades: Many people buy a used system and completely underestimate how much they'll wind up spending on it in upgrades. Sure, that $300 used laptop may look like a bargain, but once you buy a $100 external hard drive, a $90 Windows 7 license, and any programs you need to buy, it's not looking like much of a deal anymore. Heaven forbid you plan to upgrade the hardware in the immediate future—the cost of a new SSD alone on top of the purchase price may make buying new a more attractive option.

Take a Test Drive

If you're buying from a private seller, like a friend or a Craigslist contact, see if you can get a test drive of the system for a while—it might be difficult to negotiate, but a short period of use with the system, even a few hours, can make the difference between knowing the thing will boot and hold a charge and not knowing at all. If you can't get away with it for a while (and honestly, not every seller would be willing to let you walk off with the computer they're selling before paying for it) at least take some time to turn it on and give it a good look. A few things to check:

Check the frame and body for cracks, dings, or surface damage.

Check the screen for haze, dead pixels, or discoloration.

Check the inputs, ports, and optical drive for any broken or malfunctioning ones. Bring peripherals with you if possible.

Check the included software, and if anything licensed is included, make sure the keys or media are available to you.

Check the keyboard and trackpad for signs of heavy use, wear, or any broken keys

Remember: Caveat Emptor

Buying a used or old system doesn't have to be a bad experience, but you do have to be cautious when shopping. There are plenty of bargains to be had on Criagslist, eBay, and even from your friends looking to offload their old systems, but just make sure you're not throwing your money away. Some computer models have specifically high resale values, so make sure to do your homework long before arranging to meet, and make sure you wouldn't necessarily get a better deal if you bought new or refurbished. Photo by Yi Chen.